This invention relates to a fluid transportation system in which a plurality of reservoirs are connected by pipelines. More particularly, it relates to a system which controls flow rates in pipelines highly precisely so as to hold the water level of a reservoir at a desired value while satisfying the amount of demand for fluid.
As the fluid transportation systems, there are mentioned a water supply system, a gas supply system, etc. Hereunder, in order to make the description concrete, the water supply system will be taken as an example.
With increase in the amount of demand for water, the margin of the water-feed capability has lessened and the working conditions of the water supply system have become severer year after year. It has accordingly become difficult to meet the severe working conditions with a method wherein a skilled operator empirically controls the water supply every moment on the basis of the operation data of the water supply system in the past and the measured flow rate of each pipeline as well as the predicted amount of demand.
In order to overcome the difficulty, some control methods for the water supply system have been proposed. These methods, however, disregard the water-level fluctuations of a reservoir. Disadvantageously, therefore, the flow rate error becomes large, and it is impossible to schedule the water feed and control the flow rate at high precision according to the demand.
Especially, in a system wherein water is supplied from a reservoir to, for example, another reservoir by the gravitational flow, a fluctuation in the difference between both the water levels represented by the heights above the sea level has an unnegligible influence on the flow rate. FIG. 1A shows a gravitated flow unit, in which water at a flow rate Q.sub.ij is supplied from one reservoir 1-i to the other reservoir 1-j through a pipeline 2-ij as well as a valve V.sub.ij. As illustrated in FIG. 1B, when the water-level difference h.sub.ij =H.sub.i -H.sub.j between both the reservoirs 1-i and 1-j has changed to h'.sub.ij, the flow rate changes from the value Q.sub.ij to a value Q'.sub.ij even if the degree of opening of the valve V.sub.ij remains unchanged. In FIG. 1B, a solid line l and a dotted line l' are head loss--flow rate characteristic curves at different degrees of opening of the valve, respectively. It has heretofore been common practice that, even if the water-level difference h.sub.ij has fluctuated, water at a fixed flow rate is deemed to flow when the degree of opening of the valve is constant. Therefore, an error from an actual flow rate has developed, and a highly precise control of the water supply has been impossible.
Also in case of a pumped flow unit wherein, as shown in FIG. 2A, water is raised from a reservoir 1-i to another reservoir 1-j with a pump P.sub.ij, the fluctuation of a water-level difference h.sub.ij to h'.sub.ij changes the flow rate as illustrated in FIG. 2B. In the discharge pressure--flow rate characteristics of the pump indicated in FIG. 2B, a solid line l can be altered to a dotted line l' by controlling a running condition of the pump such as the number of revolutions.